Wireline formation testers, tools for the extraction of formation fluids from the wall of an open borehole full of mud, have been known for many years; and tools of this class are used extensively in oil and gas exploration. Typically, a tool of this type includes a fluid entry port, or tubular probe cooperatively arranged with a wall-engaging pad, or packer, which is used for isolating the fluid entry port, or tubular probe from the drilling fluid, mud, or wellbore fluids during the test. The tool, in operating position, is stabilized via the packer mechanism within the wellbore with the fluid entry port, or tubular probe, pressed against the wall of the subsurface formation to be tested. Gas, or other fluid, or both, is passed from the tested formation into the fluid entry port, or tubular probe via a flowline to a sample chamber of defined volume and collected while the pressure is measured by a suitable pressure transducer. Measurements are made and the signals electrically transmitted to the surface via leads carried by the cable supporting the tool. Generally, the fluid pressure in the formation at the wall of the wellbore is monitored until equilibrium pressure is reached, and the data is recorded at the surface on analog or digital scales, or both.
The tools in present use have generally performed satisfactorily in measuring formation pressures, and permeability determinations, when testing medium permeability, consolidated formations. This is not the case however, when testing tight (low permeability) or unconsolidated (very high permeability) formations. Clay particles, naturally occurring or introduced by the drilling fluid, exist in the wall pore space. In low permeability formations, these particles often adversely affect tests run at conventional flow rates by blocking the pore throats. In a tight zone, high permeability streaks release fluids and produce a buildup. The chances of setting the tool in a tight spot are always large, resulting in a "dry" test. The flowing pressure drops rapidly to zero and stays there. Most of the time no buildup occurs. When a slow buildup is recorded, starting next to zero pressure, it may be the result of fluid flow from the formation, but most likely it is due to a small leakage between the pad and the sandface. The pressure creeps slowly up, with a buildup-like shape, since the leakage decreases with the differential pressure between the borehole and the flowline. If the tester is left in place long enough, the pressure may go up to mud pressure. Needless to say, the buildup curve is meaningless in such a case.
Tight formation testing is also complicated by the supercharging effect. The mud filtrate, which is forced through the mudcake, is injected into the formation. This injection of mud filtrate causes a pressure buildup in the formation. The sandface pressure, pressure measured immediately behind the mudcake, may exceed the formation pressure by up to several hundred psi depending on the mudcake and formation permeabilities.
In medium permeability, shaly formations, damage due to both mud filtrate invasion and drilling fluid small particles invasion may render the invaded zone quasi-impervious. The filtrate damaged zone may extend several feet deep, and the particles damaged zone, up to 1/8th of an inch deep. Such a formation behaves like a tight zone; dry tests, slow buildups, and pad leakage may be experienced.
In very soft formations such as those encountered in the Gulf Coast, even when using sophisticated snorkel tubes and filters, the formation craters during the flow period and the seal is lost. The pressure in the flowline jumps to hydrostatic mud pressure and no buildup is recorded.